Tim, I think there are actually 3 loops.
MK resort loop stops at the Poly, GF, MK, Contemporary and T&TC.
Express loop stops only at MK and T&TC.
Epcot loop goes between TT&C and Epcot.
I guess the MK resort loop would take something like 10 to 15 minutes depending on stops at resorts. It can be a fun thing to do especially if you combine it with resort hopping.
Kev